Chapter 6: Functions - FAMUcop3014cjoe/Lectures/Gaddis_LectureNotes/Chap… · Starting Out with C++ Early Objects Seventh Edition ... statements that perform the function’s task

Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

Chapter 6: Functions

Starting Out with C++

Early Objects

Seventh Edition

by Tony Gaddis, Judy Walters,

and Godfrey Muganda

Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

Topics

6.1 Modular Programming

6.2 Defining and Calling Functions

6.3 Function Prototypes

6.4 Sending Data into a Function

6.5 Passing Data by Value

6.6 The return Statement

6.7 Returning a Value from a Function

6.8 Returning a Boolean Value

6-2


Topics (continued)

6.9 Using Functions in a Menu-Driven Program

6.10 Local and Global Variables

6.11 Static Local Variables

6.12 Default Arguments

6.13 Using Reference Variables as Parameters

6.14 Overloading Functions

6.15 The exit() Function

6.16 Stubs and Drivers

6-3


6.1 Modular Programming

• Modular programming: breaking a program

up into smaller, manageable functions or

modules

• Function: a collection of statements to

perform a specific task

• Motivation for modular programming

– Simplifies the process of writing programs

– Improves maintainability of programs

6-4


6.2 Defining and Calling Functions

• Function call: statement that causes a

function to execute

• Function definition: statements that make

up a function

6-5


Function Definition

• Definition includes

name: name of the function. Function names

follow same rules as variable names

parameter list: variables that hold the values

passed to the function

body: statements that perform the function’s task

return type: data type of the value the function

returns to the part of the program that called it

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Function Definition

6-7


Function Header

• The function header consists of

– the function return type

– the function name

– the function parameter list

• Example:

int main()

• Note: no ; at the end of the header

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Function Return Type

• If a function returns a value, the type of

the value must be indicated

int main()

• If a function does not return a value, its return type is voidvoid printHeading()

{

cout << "\tMonthly Sales\n";

}

6-9


Calling a Function

• To call a function, use the function name followed by () and ;

printHeading();

• When a function is called, the program executes the body of the function

• After the function terminates, execution resumes in the calling module at the point of call

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Calling a Function

• main is automatically called when the

program starts

• main can call any number of functions

• Functions can call other functions

6-11


6.3 Function Prototypes

The compiler must know the following

about a function before it is called

– name

– return type

– number of parameters

– data type of each parameter

6-12


Function Prototypes

Ways to notify the compiler about a

function before a call to the function:

– Place function definition before calling

function’s definition

– Use a function prototype (similar to the

heading of the function

• Heading: void printHeading()

• Prototype: void printHeading();

6-13


Prototype Notes

• Place prototypes near top of program

• Program must include either prototype or full function definition before any call to the function, otherwise a compiler error occurs

• When using prototypes, function definitions can be placed in any order in the source file. Traditionally, main is placed first.

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6.4 Sending Data into a Function

• Can pass values into a function at time of callc = sqrt(a*a + b*b);

• Values passed to function are arguments

• Variables in function that hold values passed as arguments are parameters

• Alternate names:

– argument: actual argument, actual parameter

– parameter: formal argument, formal parameter

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Parameters, Prototypes,

and Function Headings

• For each function argument,

– the prototype must include the data type of each parameter in its ()

void evenOrOdd(int); //prototype

– the heading must include a declaration, with variable type and name, for each parameter in its ()

void evenOrOdd(int num) //heading

• The function call for the above function would look like this: evenOrOdd(val); //call

6-16


Function Call Notes

• Value of argument is copied into parameter when the function is called

• Function can have > 1 parameter

• There must be a data type listed in the prototype () and an argument declaration in the function heading () for each parameter

• Arguments will be promoted/demoted as necessary to match parameters

6-17


Calling Functions with Multiple Arguments

When calling a function with multiple arguments

– the number of arguments in the call must match the function prototype and definition

– the first argument will be copied into the first parameter, the second argument into the second parameter, etc.

6-18


Calling Functions with

Multiple Arguments Illustration

displayData(height, weight); // call

void displayData(int h, int w)// heading

{

cout << "Height = " << h << endl;

cout << "Weight = " << w << endl;

}

6-19


6.5 Passing Data by Value

• Pass by value: when argument is passed to a function, a copy of its value is placed in the parameter

• Function cannot access the original argument

• Changes to the parameter in the function do not affect the value of the argument in the calling function

6-20


Passing Data to Parameters by Value

• Example: int val = 5;evenOrOdd(val);

• evenOrOdd can change variable num, but

it will have no effect on variable val

6-21

5

val

argument in

calling function

5

num

parameter inevenOrOdd function


6.6 The return Statement

• Used to end execution of a function

• Can be placed anywhere in a function– Any statements that follow the return

statement will not be executed

• Can be used to prevent abnormal termination of program

• Without a return statement, the function ends at its last }

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6.7 Returning a Value From a Function

• return statement can be used to return a value from the function to the module that made the function call

• Prototype and definition must indicate data type of return value (not void)

• Calling function should use return value, e.g., – assign it to a variable– send it to cout– use it in an arithmetic computation– use it in a relational expression

6-23


Returning a Value – the return

Statement

• Format: return expression;

• expression may be a variable, a literal value, or an expression.

• expression should be of the same data type as the declared return type of the function (will be converted if not)

6-24


6.8 Returning a Boolean Value

• Function can return true or false

• Declare return type in function prototype and heading as bool

• Function body must contain return

statement(s) that return true or false

• Calling function can use return value in a

relational expression

6-25


Boolean return Example

bool isValid(int); // prototype

bool isValid(int val) // heading{

int min = 0, max = 100;if (val >= min && val <= max)

return true;else

return false;}

if (isValid(score)) // call…

6-26


6.9 Using Functions in a Menu-Driven

Program

Functions can be used

• to implement user choices from menu

• to implement general-purpose tasks

- Higher-level functions can call general-purpose

functions

- This minimizes the total number of functions

and speeds program development time

6-27


6.10 Local and Global Variables

• local variable: defined within a function or

block; accessible only within the function or

block

• Other functions and blocks can define

variables with the same name

• When a function is called, local variables in

the calling function are not accessible from

within the called function

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Local and Global Variables

• global variable: a variable defined outside all functions; it is accessible to all functions within its scope

• Easy way to share large amounts of data between functions

• Scope of a global variable is from its point of definition to the program end

• Use sparingly

6-29


Local Variable Lifetime

• A local variable only exists while its

defining function is executing

• Local variables are destroyed when the

function terminates

• Data cannot be retained in local

variables between calls to the function in

which they are defined

6-30


Initializing Local and Global Variables

• Local variables must be initialized by the programmer

• Global variables are initialized to 0

(numeric) or NULL (character) when the

variable is defined

6-31


Global Variables – Why Use Sparingly?

Global variables make:

• Programs that are difficult to debug

• Functions that cannot easily be re-used in

other programs

• Programs that are hard to understand

6-32


Local and Global Variable Names

• Local variables can have same names as global variables

• When a function contains a local variable

that has the same name as a global

variable, the global variable is unavailable

from within the function. The local definition

"hides" or "shadows" the global definition.

6-33


6.11 Static Local Variables

• Local variables– Only exist while the function is executing

– Are redefined each time function is called

– Lose their contents when function terminates

• static local variables– Are defined with key word static

static int counter;

– Are defined and initialized only the first time the function is executed

– Retain their contents between function calls

6-34


6.12 Default Arguments

• Values passed automatically if arguments are missing from the function call

• Must be a constant declared in prototype

void evenOrOdd(int = 0);

• Multi-parameter functions may have default

arguments for some or all of them

int getSum(int, int=0, int=0);

6-35


Default Arguments

• If not all parameters to a function have default values, the ones without defaults must be declared first in the parameter listint getSum(int, int=0, int=0);// OK

int getSum(int, int=0, int); // wrong!

• When an argument is omitted from a function call, all arguments after it must also be omittedsum = getSum(num1, num2); // OK

sum = getSum(num1, , num3); // wrong!

6-36


6.13 Using Reference Variables as

Parameters

• Mechanism that allows a function to work with the original argument from the function call, not a copy of the argument

• Allows the function to modify values stored in the calling environment

• Provides a way for the function to ‘return’ more than 1 value

6-37


Reference Variables

• A reference variable is an alias for another variable

• Defined with an ampersand (&)

void getDimensions(int&, int&);

• Changes to a reference variable are made to the variable it refers to

• Use reference variables to implement passing parameters by reference

6-38


Pass by Reference Example

void squareIt(int &); //prototype

void squareIt(int &num)

{

num *= num;

}

int localVar = 5;

squareIt(localVar); // localVar now

// contains 25

6-39


Reference Variable Notes

• Each reference parameter must contain &

• Argument passed to reference parameter must be a variable (cannot be an expression or constant)

• Use only when appropriate, such as when the function must input or change the value of the argument passed to it

• Files (i.e., file stream objects) should be passed by reference

6-40


6.14 Overloading Functions

• Overloaded functions are two or more

functions that have the same name, but different

parameter lists

• Can be used to create functions that perform the

same task, but take different parameter types or

different number of parameters

• Compiler will determine which version of function

to call by argument and parameter list

6-41


Overloaded Functions Example

If a program has these overloaded functions,void getDimensions(int); // 1

void getDimensions(int, int); // 2

void getDimensions(int, float); // 3

void getDimensions(double, double);// 4

then the compiler will use them as follows:int length, width;

double base, height;

getDimensions(length); // 1

getDimensions(length, width); // 2

getDimensions(length, height); // 3

getDimensions(height, base); // 4

6-42


6.15 The exit() Function

• Terminates execution of a program

• Can be called from any function

• Can pass a value to operating system to indicate status of program execution

• Usually used for abnormal termination of program

• Requires cstdlib header file

• Use carefully

6-43


exit() – Passing Values to Operating

System

• Use an integer value to indicate program

status

• Often, 0 means successful completion,

non-zero indicates a failure condition

• Can use named constants defined in cstdlib:

– EXIT_SUCCESS and

– EXIT_FAILURE

6-44


6.16 Stubs and Drivers

• Stub: dummy function in place of actual function

• Usually displays a message indicating it was called. May also display parameters

• Driver: function that tests a function by calling it

• Stubs and drivers are useful for testing and debugging program logic and design

6-45

Copyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-WesleyCopyright © 2011 Pearson Education, Inc. Publishing as Pearson Addison-Wesley

Chapter 6: Functions

Starting Out with C++

Early Objects

Seventh Edition

by Tony Gaddis, Judy Walters,

and Godfrey Muganda

Documents

Chapter 6: Functions - FAMUcop3014cjoe/Lectures/Gaddis_LectureNotes/Chap… · Starting Out with C++ Early Objects Seventh Edition ... statements that perform the function’s task